The present invention relates generally to the field of magnetic resonance imaging (MRI) and, in certain particular embodiments, to techniques for reducing coupling in an array of radio-frequency receiver coils and/or transmitter coils used in parallel MRI devices, for instance.
MRI is a non-invasive imaging technique in which the imaged subject is placed in a static main magnetic field known as the “B0” field. While in the B0 field, gradient coils are excited to create the “B1” field, in turn, exciting the nuclei of the imaged subject. In certain MRI systems, a plurality of radio-frequency coils are needed to transmit the RF energy to the nuclear magnetic moments as well as to receive the extremely small signal that comes back from the subject. The signals, referred to as magnetic resonance signals, result from reorientation of certain gyromagnetic materials of the subject, whose molecules spin or precess at characteristic frequencies. The radio-frequency coils are commonly employed to image whole body, head and limb imaging in medical applications. Moreover, MRI devices often include an array of receiver coils to improve the performance of the MRI devices. Irrespective of the specific implementation, MRI techniques are also used outside of the medical imaging field, such as in part inspection, baggage inspection, and so forth.
In recent years, parallel MRI imaging has emerged as an attractive imaging modality that results in scan time reduction, resolution enhancement, artifact suppression, and, even, attenuation of noise. In a general sense, parallel imaging utilizes the difference in sensitivities between individual coils of a receiver array to reduce the number of gradient encoding steps required for imaging. Thus, in parallel MRI, an array of receiver coils with different sensitivities is used to receive the signal in parallel, facilitating combination of these obtained signals to reconstruct the full image.
There are several parallel MRI approaches, including SMASH (SiMultaneous Acquisition of Spatial Harmonics) and SENSE (SENSitivity Encoding). For pulse sequences that execute a rectilinear trajectory in k space, these techniques reduce the number of phase encoding steps in order to reduce imaging time, and then use array sensitivity information to make up for the loss of spatial information.
In order to make the coils in the array sufficiently spatially distinct, and thus improve their signal-to-noise ratio (SNR) for accelerated imaging, it is common to leave gaps between neighboring coils in the array. This, however, has the drawback of increasing the coupling between coils, which can in turn degrade performance. To overcome this limitation, the coils in the array may be overlapped by an amount that minimizes the mutual inductance between neighboring coils. When such an array is employed for parallel imaging, the SNR decreases, because the geometry factor of the array has increased. The geometry factor quantifies the spatial distribution of the noise amplification. The optimization of array geometry is necessary to minimize the geometry factors in MRI.
It would therefore be desirable to have a technique to reduce the coupling between coils in parallel imaging arrays without compromising the SNR.